Ten years ago today, the world saw what was by some accounts the largest single coordinated protest in history. Roughly 10 million to 15 million people (estimates vary widely) assembled and marched in more than 600 cities: as many as 3 million flooded the streets of Rome; more than a million massed in London and Barcelona; an estimated 200,000 rallied in San Francisco and New York City. From Auckland to Vancouver — and everywhere in between — tens of thousands came out, joining their voices in one simple, global message: no to the Iraq war.

I was among the antiwar contingent that swarmed Manhattan’s midtown on Feb. 15, 2003, a wintry Saturday. We spread across miles of city blocks, trundling past abandoned police barricades as we tried to inch toward the U.N., where 10 days earlier then Secretary of State Colin Powell had presented what we now know was illusory intelligence about Iraq’s supposed weapons of mass destruction. The multitudes in New York were diverse and legion. There were anarchists and military veterans, vociferous students (I was then a freshman in college) and a motley cast of graying peaceniks — many, including one grandmother memorably puttering along in a wheelchair, had opposed American involvement in Vietnam. And there were myriad others: a band of preppy suburbanites with banners announcing themselves — “Soccer Moms Against the War” — musicians, street artists and workaday New Yorkers. My uncle, a doctor with medical practices in both the U.K. and India, had flown in for the demonstration and was just another face in a vast crowd.

The overwhelming feeling on New York’s streets, despite the grimness of the NYPD and the bite of that February afternoon, was one of unity and hope. Word was seeping in about the scale of the demonstrations elsewhere and it was hard not to bask in our sense of collective purpose. An article in the New York Times would soon trumpet, “There are two superpowers: the United States and world public opinion.” Here’s Sofia Fenner, then a high school senior in Seattle (now a doctoral candidate at the University of Chicago, currently doing dissertation work in Cairo): “I was just proud to stand with all those people, proud that we as dissenting Americans were not staying home while what seemed like the whole world took up our cause.” In Los Angeles, a pregnant Laila Lalami walked a mile with fellow protesters down Hollywood Boulevard. “I thought, ‘Hundreds of thousands of people across the U.S. are making their voices heard. Surely they can’t be ignored,’” the Moroccan-American novelist told TIME this week. “But they were.”

And there it was. We failed. Slightly more than a month later, the U.S. was shocking and awing its way through Iraqi cities and Saddam Hussein’s defenses and bedding in — though it didn’t know it yet — for a near decadelong occupation. The protests, which by any measure were a world historic event, were brushed aside with blithe nonchalance by the Bush Administration and a rubber-stamp Congress that approved the war. The U.N.’s Security Council was bypassed, and the largely feckless, acquiescent American mainstream media did little to muffle Washington’s drumbeats of war.

A decade later, it’s hard to understand why the display of people power on Feb. 15 proved so ineffectual. The gun-slinging righteousness of post-9/11 America has given way to a more humble West, burdened by unwinnable wars, financial crises and a semipermanent funk of political dysfunction. Moreover, the explosion of social media in recent years has enabled previously obscure episodes of dissent to reach and reshape the global conversation. Protests matter again. Public spaces — from Cairo’s Tahrir Square to Madrid’s Puerta del Sol to New York’s tiny Zuccotti Park — became sites of a renewed democratic vitality. Yet the mass antiausterity protests that have rocked Europe or even the largest actions of Occupy Wall Street have not been able to match the scale of what took place on Feb. 15, 2003.

There will be time yet to relitigate the justifications behind the U.S.-led invasion of Iraq, 10 years after the fact. The ranks of the war’s cheerleaders have thinned in the intervening years, with a host of journalists and pundits in the U.S. offering their mea culpas for supporting the war so unquestioningly. A dictator is gone, but more than 100,000 Iraqis are dead, as well as 4,804 U.S. and coalition soldiers. The U.S. spent nearly a trillion dollars on a pre-emptive war that didn’t need to happen and a nation-building exercise that has achieved only fragile, uncertain gains. Far from a “mission accomplished,” the American adventure in Iraq has become a cautionary tale of hubris and poor planning. It’s clear the West’s current reluctance to take more direct action in ending Syria’s bloody civil war is, in part, a legacy of the U.S. experience in Iraq, where the disintegration of a regime spawned a whole new phase of sectarian slaughter and chaos.

But there’s no satisfaction in looking back and saying, “I told you so” — not with the blood that has been spilled and continues to be spilled. That profound solidarity I felt 10 years ago has faded into a form of resignation and sadness. In a region as complex and politically volatile as the Middle East, fixed moral positions are difficult. “Our demands were simple [on Feb. 15], and we were right,” says Fenner, the University of Chicago doctoral candidate. “What I didn’t realize at the time was that when the war went ahead, nothing would ever be so simple again.”